There was never much doubt over the outcome, whatever the chaos in the air, whatever the vagaries of the home team’s form. English champions at home to a side renowned, even by the standards of the French, for their travel sickness is a fixture that should not require a shaman to divine.

So it turned out. Northampton emerge from the floodlit, swirling rainstorm with the full complement of points, six tries and the successful reintegration of Alex Mitchell into their starting lineup. It would be misleading to describe this as a return to their champion form of last season, just as it would be unfair to describe Castres as waving them through obligingly, but this was virtually the definition of a job done with the minimum of fuss.

Related: Bath comeback thwarted as La Rochelle sparkle in the rain in first-half blitz

Mitchell grew into the match after a couple of early errors, which might have been rust or possibly the horizontal rain in his face, and there were authoritative performances from those in his vicinity at the heart of the team. Juarno Augustus, off to Ulster at the end of this season, was a huge presence in the back row alongside young Henry Pollock, similarly rambunctious and not going anywhere any time soon if Northampton have anything to do with it. Meanwhile, Fin Smith played with assurance in the challenging conditions.

Not that anyone will be getting carried away. Castres have not won on the road this season, even though they are unbeaten at home. The number of changes they made did not bode well. Injuries were cited as the reason, which may or may not be fair enough. Such is Castres’ history for focusing on their domestic campaign that no one was expecting much in the way of existential desperation.

They did not quite roll over, but Saints notched up two tries in the first 10 minutes, despite a gale against them. Fraser Dingwall picked a simple line for the first, after Smith’s cross-kick had Castres in all sorts of trouble. That was in the fourth minute. Castres did respond after Mitchell spilled the restart, Louis Le Brun kicking a penalty.

Exeter fell to a 39-21 defeat against the Sharks in Durban. The Chiefs took an early lead through Dan Frost’s try while they were down to 14 with Greg Fisilau in the sin-bin. Frost claimed a second score at the end of the first half, but by then the Challenge Cup holders had crossed four times themselves. Phepsi Buthelezi, Makazole Mapimpi, Grant Williams and Siya Kolisi dotted down for a 26-14 interval lead.

Kolisi’s second try stretched the Sharks’ lead before England winger Immanuel Feyi-Waboso claimed a crossfield kick for Exeter’s third score. Despite the Sharks being reduced to 12 men for about six minutes later with three players sent to the sin-bin in quick succession, Exeter could not score a fourth try and register a bonus point.

Harlequins and Sale both lost away, with the London club going down 23-12 at Racing 92. The French hosts seized control with 11 unanswered points in the final quarter after Alex Dombrandt and Will Porter scored tries for Harlequins and Marcus Smith added a conversion.

The Glasgow scrum-half George Horne claimed a first-half hat-trick in their 38-19 victory over Sale at Scotstoun Stadium. Kyle Rowe, Huw Jones and Scott Cummings were also on the try sheet, while Sale replied through Arron Reed, Joe Carpenter and a penalty try.

Stade Français played with 13 men for the final 27 minutes of their 33-7 defeat at Munster. Pierre Azagoh was sent off for catching Peter O'Mahony with a flying arm and Baptiste Pesenti saw red nine minutes later for a dangerous tackle. Munster secured a bonus point with tries from Thaakir Abrahams, Shane Daly, Alex Kendellen and Tom Farrell.

Clermont Auvergne shut out Benetton in a 28-0 victory Stade Marcel-Michelin. Peceli Yato and Barnabe Massa both crossed twice with Benjamin Urdapilleta converting all four tries.

Toulon overcame a 14-10 interval deficit to record an impressive 24-14 success away to the Stormers in Cape Town. Gabin Villière, Facundo Isa and Baptiste Serin crossed for the visitors, while Warrick Gelant and Joseph Dweba scored tries for the South African hosts. PA Media

Smith caught Castres unaware with a quick restart after that, before Temo Mayanavanua chose a more outrageous line to saunter through the Castres fringe defence. And so the scoring concluded for the half. Pollock thought he had scored two minutes later again, sliding and gathering over the line, but he just lost control as he went over. Meanwhile, Josaia Raisuqe missed a simpler opportunity for Castres he had himself created, when he crossed the line but failed to take the ball with him.

Northampton had the elements in their favour for the second half. Two further tries accrued, again in the first 10 minutes. Smith’s chip and follow-up had Castres offending, before Augustus called for the ball out wide. Tommy Freeman tapped the penalty and found the No 8 for the first of his brace. Five minutes later, Pollock and Augustus scattered various bodies, and Mitchell managed to put Curtis Langdon over via Lois Guérois-Galisson’s flailing hand.

Bonus point and match sewn up. Nobody could blame the ever-emptying crowd after that, such was the storm whipped into their faces.

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Augustus was held up just after the hour, but he scored anyway from the next play to clinch the match award. Pierre Colonna, the replacement hooker, charged through in uncomplicated style for the visitors’ sole try, before Pollock, who might have earned the match award himself, gallivanted through half the Castres midfield for a try of his own.

Job done. Next up, the very different proposition, in more ways than one, of the Bulls in Pretoria.